Today, emergency calls are handled via uniform national phone numbers, such as 110 and 112 in Germany or 911 in North America. Upon dialing of these numbers, there is added to the call signal, for source identification purposes, a sequence of numbers that corresponds to the respective local network code (ONKZ). In accordance with this source or location code, i.e. the physical origin of an emergency call, the responsible exchange addresses the respective geographically responsible emergency call center, associated with the exchange.
Although thus far, due to a fixed association between the respective telephone number and an exchange at a specific location, the calling number already contains a clear position identification, in the future, due to allocation of telephone numbers among multiple different service providers, an association of the location of the respective terminal with the calling number will no longer be possible. There will also be TC subscribers who will want to take their phone number with them when they move, within a local network for example. This will make the determination of the correct geographically responsible emergency call center more difficult or impossible, if covered numbering is specified by the position of the exchanges in the existing local network, and the subscriber moves to another part of the city, with a different identification. Even in these cases, it must be ensured that the emergency call from an individual subscriber is forwarded to another, namely the geographically responsible, local emergency call center. Furthermore, there may be exchanges which must address several emergency call centers in different local networks.
In a forwarding method known from DE 35 90 468 T1 and U.S. Pat. No. 4,689,811, if any emergency call center has a night service connection or is out of service for any reason, a call from a TC subscriber is automatically diverted to another emergency call center. The subscriber's TC installation is programmed so that an emergency call goes to a predetermined first emergency call center. To that end, a code corresponding to the number of the first emergency call center is normally stored in the subscriber's installation. If, when a call is made, no acknowledgment signal is received from the first emergency call center, after a specified waiting period, routing to another predetermined second emergency call center is made. In case of a relocation of a subscriber, this routing method would require a corresponding change in the preprogrammed emergency call centers and their sequence.
A system for transmitting emergency calls is also known from DE 33 21 416 C2, RUPP, in which the emergency call contains an identification of the subscriber's device. Since the assignment of the identifications of the call numbers of the limited emergency call subscriber devices is stored in a call number memory, it can be detected, on the basis of this identification, from which of the many emergency call subscriber devices the call originates. However, in this transmission system as well, the emergency calls are routed, in accordance with their local network identification, to the associated respective geographically responsible emergency call centers.
Furthermore a method of transmitting emergency calls is known from U.S. Pat. No. 4,872,194 TRELL, whereby, when an emergency call is made, corresponding emergency call centers are dialed with the help of a discriminator. In this case, the selection of the respective responsible emergency call center takes place by means of the respective dialed telephone number.
Finally, a telephone alarm system is known from U.S. Pat. No. 4,051,327, RIGSBY, in which an encoded word information is stored in the memory of a transmitting or calling unit, which clearly identifies the transmitting or calling unit and also stores information such as, e.g., the telephone number of the receiving unit.